1. Field of the Invention
The present invention relates to a two stage process useful for cetane upgrading of diesel fuels. More particularly, the invention relates to a process for selective naphthenic ring-opening utilizing an extremely low acidic distillate selective catalyst having highly dispersed Pt. The process is a two stage process wherein the first stage is a hydrotreating stage for removing sulfur from the feed and the second stage is the selective ring-opening stage.
2. Description of Prior Art
Under present conditions, petroleum refineries are finding it increasingly necessary to seek the most cost-effective means of improving the quality of diesel fuel products. Cetane number is a measure of ignition quality of diesel fuels. Cetane number is highly dependent on the paraffinicity of molecular structures whether they be straight chain or alkyl attachments to rings. Distillate aromatic content is inversely proportional to cetane number while a high paraffinic content is directly proportional to a high cetane number.
Currently, diesel fuels have a minimum cetane number of 45. But the European Union (EU) just passed an amendment requiring that the cetane number of European diesel fuels reach 51 by the year 2000, even higher cetane numbers of at least 58 are being proposed for the year 2005 and beyond.
Aromatic compounds are a high source of octane, but they are poor for high cetane numbers. Aromatic saturation, which can be described as the hydrogenation of aromatic compounds to naphthene rings, has been commonly used to upgrade the cetane level of diesel fuels. However, aromatic saturation can only make low cetane naphthenic species, not high cetane components such as normal paraffins and iso paraffins. As a result, the use of a hydrocracking catalyst for the ring-opening of naphthenic species had been used to solve this problem.
Conventional hydrocracking catalysts that open naphthenic rings rely on high acidity to catalyze this reaction. Because hydrocracking with a highly acidic catalyst breaks both carbon-carbon and carbon-hydrogen bonds, the use of such a catalyst cannot be selective in just opening rings of naphthenic species without cracking desired paraffins for the diesel product.
Furthermore, commercial hydrocracking catalysts rely on acidity as the active ring-opening site, and this active site also catalyzes increased hydroisomerization of the resulting naphthenes and paraffins. It is typical for a cumulative loss of 18-20 cetane numbers for each methyl branching increase. The use of a low acidic catalyst would minimize diesel yield loss, the production of isoparaffins, and the production of gaseous by-products.
Hydroprocessing can be done in a co-current, counter-current or an ebullated bed configuration. In a conventional co-current catalytic hydroprocessing, a hydrocarbon feed is initially hydrotreated to help get rid of heteroatom-containing impurities. These heteroatoms, principally nitrogen and sulfur, are converted by hydrodenitrogenation and hydrodesulfurization reactions from organic compounds to their inorganic forms (H2S and NH3). These inorganic gases inhibit the activity and performance of hydroprocessing catalysts through competitive adsorption on the catalyst. Therefore, the catalyst containing portion of a conventional co-current reactor is often limited in reactivity because of low H2 pressure and the presence of high concentrations of heteroatom components.
Conventional counter-current configurations utilizes a device that creates a flow of hydrogen containing gas within a container in order to force the gaseous phase to flow counter to the liquid phase. U.S. Pat. No. 5,888,376 discloses a counter current process for converting light oil to jet fuel by first hydrotreating the light oil and then flowing the product stream counter-current to upflowing hydrogen containing gas in the presence of hydroisomerization catalysts. These hydroisomerizaton catalysts are highly acidic catalysts. U.S. Pat. No. 5,882,505 also discloses hydroisomerizing wax feedstocks to lubricants in a reaction zone containing an acidic hydroisomerization catalyst in the presence of a hydrogen-containing gas. U.S. Pat. No. 3,767,562 discloses making jet fuel by using a hydrogenation catalyst in a counter-current configuration. None of the counter-current methods in the prior art discloses the use of a catalyst that can selectively open naphthenic species without cracking desired paraffins.
In light of the disadvantages of the conventional processes for improving diesel fuel, there remains a need for a process of selective naphthenic ring-opening that produces an increased cetane number of diesel fuel without a corresponding diesel yield loss.
In accordance with the present invention, a process is provided for selective ring-opening of naphthenes catalyzed by a low acid catalyst in order to increase diesel fuel yield and cetane number.
In the process, a hydrocarbon feed is contacted with a hydrogen containing gas under superatmospheric conditions with a selective ring-opening (SRO) catalyst. Ideally, the process operates in a counter-current configuration in order to remove gaseous heteroatoms. In the counter-current configuration, the catalyst can operate at lower temperatures in order to minimize hydrocracking and hydroisomerization of paraffin, thereby increasing cetane number and diesel yield. The selective ring-opening catalyst preferably has a crystalline molecular sieve material component and a Group VIII noble metal component. The crystalline molecular sieve material component is a large pore faujasite structure having an alpha acidity of less than 1, preferably less than 0.3. Zeolite USY is the preferred crystalline molecular sieve material component.
The Group VIII noble metal component can be platinum, palladium, iridium, rhodium, or a combination thereof. Platinum is preferred. The content of Group VIII noble metal component can vary. The preferred range is between 0.1 and 5% by weight of the catalyst.
The Group VIII noble metal component is located within the dispersed clusters. In the preferred embodiment, the particle size of Group VIII metal on the catalyst is less than about 10 xc3x85. Dispersion of the metal can also be measured by hydrogen chemisorption techniques in terms of the H/metal ratio. In the preferred embodiment, when platinum is used as the noble metal component, the H/Pt ratio is between about 1.1 and 1.5.
The advantages of the present invention is that (1) it allows selective ring opening of naphthene rings by the use of a low acid catalyst in addition to hydrogenating aromatics and cracking heavy paraffins, and (2) it allows the low acid catalyst to operate at the lowest possible temperature by using a counter-current configuration in order to prevent undesired hydrocracking and hydroisomerization.
For a better understanding of the present invention, together with other and further advantages, reference is made to the following description, taken in conjunction with accompanying drawings, and its scope will be pointed out in the appended claims.